Rotordynamic pumps, such as centrifugal, mixed-flow, and axial-flow pumps with mechanical bearings or magnetically suspended systems, have been widely used as a ventricular assist devices to support patients with heart diseases. In magnetically levitated blood pumps, which generally include an impeller that is both magnetically suspended and rotated without mechanical means, an annular gap located between the rotor and stator suspension and drive components is conventionally designed to be relatively small. A narrow annular flow gap generally necessitates higher rotational speeds of the rotor in order to generate the desired pressure rise and flow rates needed to support patients. One adverse outcome of operating a rotor at high rotational speeds is a tendency for high turbulence flow characteristics within the blood (e.g., high shear stress) that increase the extent and rate of red blood cell damage.
Additionally, for centrifugal or mixed-flow blood pumps with shrouded impellers (i.e., a circumferentially revolved surface interconnecting the impeller blade tips), the fluid within the clearance space between a rotating front shroud and the stationary housing demonstrates a complex three-dimensional structure, leading to retrograde leakage flow and strong disk friction loss. The combination of disk friction loss and the strong vortical flow not only lowers pump efficiency but also potentially induces hemolysis and thrombosis. A similar flow pattern can also occur at the back clearance space between a rotating back shroud and the stationary housing for centrifugal or mixed flow pumps with or without a front shroud. The level of shear stress within the clearance between the walls of a shroud and housing depends, at least in part, on the pump rotational speed.
For centrifugal or mixed-flow blood pumps with unshrouded or semi-open impellers, the lack of a front shroud introduces a problem due to the blade tip leakage flow from pressure-side to suction-side of the blades which occurs through the clearance between the rotating blade tip and the stationary housing. The leakage flow can also generate a jet leakage vortex that interacts with the primary flow, causing hydraulic loss and possibly inducing blood trauma. The shear stress exhibited in the gap or clearance between the blade tip gap and the stationary housing is very sensitive to the pump rotational speed as well as the magnitude of the gap itself.
For axial flow blood pumps with completely magnetically suspended systems, the annular gap located between the cylindrical rotor and housing has to be small enough to maintain the magnetic radial stiffness. Additionally, the axial length of the rotor has to be sized to maintain proper stability, exhibiting sufficient axial stiffness and little yaw. Such an arrangement generally leads to the requirement for high pump speed in order to generate the required pressure rise and flow rate for patients. However, the shear stress exhibited by the fluid within the annular gap region can become very high due to the high rotational speed and the narrowness of the gap. Moreover, conventional designs of axial blood pumps tend to have very long blade profiles (i.e., extending long axial distances and having very large blade wrap angle) and large trailing edge angles (i.e., β2 close to 90 degrees). Such a design with very long blade profiles not only increases the blade tip areas with higher shear stress but also leads to flow separation and vortices, particularly at the off-design conditions. Moreover, in addition to deterioration of the pump efficiency, such a design most likely causes undesirable blood damage.